Electrical power distribution equipments are subject to transient electrical events, such as lightning, which affect the voltages and currents generated by the equipment. Power utility companies find it necessary to monitor and analyze the effect of such events by recording and analyzing data relating to the voltages and currents developed by such transient phenomena so that proper protection and safeguards for maintaining the electrical stability of the power distribution equipment can be provided. These transient electrical events are of relatively short duration and generate high frequency voltages or currents.
The prior art approach was to sample data for short intervals at a high frequency to capture fast rise times or to sample at a lower frequency and forego the capture of the high frequency events. By way of example, a prior art system sampling a 60 Hz waveform with a high frequency additive pulse may have a memory consisting of 2,048 words of 24 bytes each, in which an analog-to-digital converter samples had a 5 MHz rate with 8 bit resolution. The fixed sampling system captures and includes high frequency components with 200 nanoseconds between sampling points. However, the total recorded time would be 1.2 milliseconds, assuming data packing of three samples per word for a total of 6144 words. In such a case, only a fraction of a single cycle at 60 Hz could be reconstructed, which which would provide little definition of the 60 Hz waveform on which the high frequency spike was superimposed. Also, in prior art systems two or three different sampling rates were utilized as a tradeoff between the two approaches.
Another alternative is to use very large memories or to vary in a predetermined manner the sampling rate referenced to a trigger point. However, large capacity memories are characterized by high power consumption, undue costs, increased complexity and a reduction in reliability, whereas fixed sampling sequences generally experience a loss of important data, such as a repeat strike of lightning.